Conformational Analysis of Guaianolide-Type Sesquiterpene Lactones by Low-Temperature NMR...

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Structural Chemistry, Vol. 15, No. 3, June 2004 (C© 2004)

Conformational Analysis of Guaianolide-Type SesquiterpeneLactones by Low-Temperature NMR Spectroscopy andSemiempirical Calculations

Slobodan Milosavljevic,1 Ivan Jurani c,1 Vanja Bulatovi c,2 Slobodan Macura,3

Nenad Juranic,3 Hans-Heinrich Limbach,4 Klaus Weisz,4 Vlatka Vajs,5

and Nina Todorovic5,6

Received June 26, 2003; revised September 11, 2003; accepted September 29, 2003

Conformational analysis of 9α-acetoxycumambrine A1 and 8-O-isobutiryl-9α-acetoxycumambrineB 2 was carried out by low-temperature NMR studies. Results suggested that lactones1 and2 aremixtures of two distinctive conformers, I and II. Based on low-temperature1H NMR spectra, infour solvents, the thermodynamic parameters of I⇀↽ II exchange process were assessed. Energy ofactivation of I→ II reaction was obtained by dynamic NMR simulations for both compounds. Resultsrevealed that conformational exchange of lactones1 and2 occurs due to “chair⇀↽ twisted chair”interconversion of a heptane ring. The same PM3 semiempirical method was applied for geometryoptimization of lactones1 and2, as well as of 9α-hydroxycumambrine A3, 9α-acetoxycumambrineB 4, and cumambrine B5.

KEY WORDS: Guaianolides; cumambrine; conformations; low-temperature NMR; dynamic NMR; PM3semiempirical calculations.

INTRODUCTION

Chemotaxonomic importance of the naturalsesquiterpene lactones, as well as their prominent anddiverse physiological activity, are the major reasons forcontinuous interest regarding these compounds. It waspointed out that their activity is most likely associatedwith the α, β-methyleneγ -lactone part of a molecule.Nevertheless, in addition to structural demands, it iswell known that geometry of the entire molecule isimportant for the expression of biological activity.

1Faculty of Chemistry, University of Belgrade, 11000 Belgrade, Serbiaand Montenegro.

2Institute for Medicinal Plant Research “Dr. Josif Panˇcic,” 11000Belgrade, Serbia and Montenegro.

3Department of Biochemistry and Molecular Biology, Mayo Foundation,Rochester, Minnesota 55905.

4Institut fur Chemie, Freie Universit¨at Berlin, Berlin, Germany.5Institute for Chemistry, Technology and Metallurgy, Njegoˇseva 12,11000 Belgrade, Serbia and Montenegro.

6To whom correspondence should be addressed; e-mail: ninat@chem.bg.ac.yu

Accordingly, elucidation of conformational properties ofthese compounds could be interesting. During the lastfew decades, an appreciable number of sesquiterpenelactones has been isolated from the plant sources andguaianolides represent one of the most abundant group.However, conformational analysis of guaianolides re-ceived relatively little attention [1] and, having in mindnumerous conformational possibilities of a heptane ring,papers about conformational exchange phenomena ofguaianolides are surprisingly rare [2].

Recently, we have reported isolation and structuredetermination of conformationally flexible guaianolides1 and 2 (Fig. 1) [2a, b]. Well-known cumambrine B5 was isolated on the same occasion [2b]. Also, as apart of our previous investigations, the inseparable struc-tural isomers3 and 4 (Fig. 1) were isolated as a mix-ture (3/4 = 15/85) and their structures were fully de-termined by NMR spectra of the mixture [2c]. TheseNMR spectra unambiguously showed that lactone4 isin conformational equilibrium, contrary to its structuralisomer3.

2371040-0400/04/0600-0237/0C© 2004 Plenum Publishing Corporation

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Fig. 1. Guaianolides1–5.

Herein we present an investigation of the conforma-tional equilibrium of lactones1 and2 by low-temperatureNMR experiments and evaluation of the conformations ofall five guaianolides1–5 (Fig. 1) by the PM3 semiempir-ical method.

EXPERIMENTAL

Guaianolides1–5 originate from plant speciesAn-themis carpaticaand their isolation and structure deter-mination were reported elsewhere [2a–c].

NMR STUDIES

The NMR spectra were measured with BrukerDRX300 (operating at 300.13 MHz for1H) and BrukerDRX500 (operating at 500.13 MHz for1H) spectrome-ters, in deuterated solvents. Coupling constants (J) aregiven in Hz and chemical shifts (δ) are reported relativeto TMS as internal standard. The 5-mm double resonanceprobeheads1H/13C were used at both spectrometers andwere kept at room temperature by an external thermo-stat, so only the sample tube was cooled down to therequired temperatures. The temperatures were adjustedby Eurotherm Variable Temperature Unit using the exter-nal devices, “thermocouple K” and “Pt-100-thermo,” forBruker DRX300 and Bruker DRX500, respectively. Alltemperatures were measured to an accuracy of±1◦C.

The conformational exchange I⇀↽ II of compounds1 and2 was investigated by two sets of1H NMR experi-ments: (1) All 1D1H NMR measurements for elucidatingthermodynamic parameters of the exchange process wereperformed with Bruker DRX300 spectrometer, in methy-lene chloride-d2, methanol-d4, acetone-d6, and toluene-d8, at six temperatures in a range of≈ 180 to 240 K.The temperature scale was calibrated with methanol bystandard procedure [3]. The line shapes of spectra below

250 K revealed a slow exchange regime; hence, those sig-nals that are not affected by impurities, or overlapped withother signals, could be reliably integrated. The equilibriumconstantsKI-II = [II]/[I] were obtained as the ratio of theintegrals of relevant, separated1H signals of conformersI and II. The signals selected for integration were: 13-H,6-H, and 7-H in CD2Cl2 and CD3OD, 13-H and 15-Hin (CD3)2CO, and 13-H and 13′-H in C7D8. (2) The se-ries of 1D1H NMR spectra for band-shape analysis wererecorded with Bruker AMX500, in CD2Cl2, over a tem-perature range of 233 to 313 K. All acquisition and pro-cessing parameters were exactly the same for compounds1 and2, except a number of scans (ns); calculated con-centrations were 2 mg cm−3 (ns= 256) and 5 mg-cm−3

(ns= 64), for compounds1 and2, respectively. Previous1H NMR experiments revealed that there was no concen-tration dependence associated with chemical shifts, linewidths, or signals intensities. Dynamic NMR simulationswere achieved using in-house7 developed programs basedon the quantum-mechanical matrix formalism of Binsch[4]. The spectrum at 233 K was treated as having a zerointerconversion rate for both compounds,1 and2, and forboth interconverting species, I and II.

Theoretical Calculations

The energy minimization of lactones1–5 was per-formed by semiempirical quantum mechanical PM3method comprised in HyperChem 4.0 program package,applying Polak–Ribiere minimization algorithm. Startingstructures were created with HyperChem and initiallyminimized to RMS gradient less than 0.05 kcal-A−1-mol−1. The conformational search was performed apply-ing a direct search method and standard settings. All rotat-able cyclic bonds were included as variable torsions andallowed to change simultaneously. Acyl side chains wereincluded in the conformational search. The resulting struc-tures were energy minimized to a RMS gradient as above.

RESULTS AND DISCUSSIONS

Conformational Analysis of 9α-AcetoxycumambrineA (1) and 8-O-Isobutiryl-9α-acetoxycumambrineB (2)

Thermodynamic Parameters of the Exchange Process

The NMR results show a high degree of analogy inconformational behavior of compounds1 and 2. Broad

7AG Limbach, Institut fur Chemie, Freie Universit¨at Berlin.

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Conformational Analysis of Guaianolide-Type Sesquiterpene Lactones 239

Fig. 2. 1H NMR spectra (CD2Cl2) of: (a) compound1, T = 214 K,I/II = 1/1.2; (b) compound2, T = 217 K, I/II = 1/2.

lines in their room-temperature NMR spectra are typi-cal for the conformational exchange of a medium rateon NMR time scale. For both lactones, coalescence pointwas atTc = 278 K and below 253 K most of1H and13C NMR signals split into pairs of sharp, well-resolved

Table I. Conformer Populations (%) for Lactones1 and2 According to1H NMR Spectra in Different Solvents atthe Ends of Temperature Range (Approximate Temperature Values)

CD2Cl2 CD3OD (CD3)2CO C7D8

Compound Conformer 180 K 240 K 180 K 240 K 180 K 240 K 180 K 240 K

1 I 56 47.6 87.8 80 80.7 75.2 80 55.9II 44 52.4 12.2 20 19.3 24.8 20 44.1

2 I 35 31.5 87.8 80.3 81.1 73.4 61.9 40.6II 65 68.5 12.2 19.7 18.9 26.6 38.1 59.4

resonances, thus indicating presence of two conformers,I and II (Fig. 2). Within each pair in PS-NOESY, the1Hsignals were connected by positive cross peaks, character-istic for slow-exchanging conformers [2(a,b)]. Apart fromthe consistent difference in chemical shifts of the corre-sponding NMR signals, the conformers exhibited differentvicinal couplings of 8-H to 7-H and 9-H:J7,8 ≈ 11, J8,9 ≈2.5 Hz andJ7,8 ≈ 9, J8,9 ≈ 5 Hz, for conformers I and II,respectively.

We have recorded1H NMR spectra of lactones1 and 2 in four solvents, in temperature range of≈180 to 240 K and conformer distributions are estimatedon the integral values of the suitable1H NMR signals(Table I).

Variable-temperature1H NMR measurements re-vealed the increasing population of conformer II at theexpense of I, with temperature increases (Table I). Appar-ently, conformer distribution virtually depends on the di-electric constant of the solvent. In polar solvents [CD3OD,(CD3)2CO], conformer I predominates over the entire tem-perature range, for1 and2. In nonpolar solvents (CD2Cl2,C7D8) conformer distribution is much more dependent ontemperature than in polar ones (Table I). From the ratios ofthe integrals of the appropriate1H NMR signals (given inparenthesis), at six temperatures (calibrated by methanol),in four solvents, equilibrium constantsKI-II = [II]/[I] forthe exchange reaction I⇀↽ II were obtained (Tables IIand III).

The thermodynamic parameters1H ◦ and1S◦ of theexchange process, obtained by linear regression analysisfrom Eq. (1), indicate that we are dealing with mixtureof the conformers of comparable stabilities (enthalpies),while free-energy change of the I⇀↽ II reaction includesconsiderable contribution of the free entropy difference(Tables IV and V).

ln K = −1G◦/RT = −1H ◦/RT+1S◦/R (1)

Presumably, the main term in, relatively large,1S◦ of theexchange process belongs to the entropy of mixing, favor-ing conformation II at higher temperatures, particularly innonpolar solvents.

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Table II. Equilibrium ConstantsKI-II for Lactone1: CD2CI2, CD3OD, (CD3)2CO, andC7D8

KI-II KI-II

Tcalib (K) (13-H) (6-H) (7-H) Tcalib (K) (13-H) (6-H) (7-H)

CD2Cl2 CD3OD180.17 0.803 0.831 0.707 180.17 0.145 0.132 a

192.65 0.886 0.950 0.774 192.65 0.147 0.148 a

203.57 0.987 1.040 0.899 204.93 0.173 0.175 0.189217.02 1.070 1.111 0.980 218.35 0.192 0.208 0.215228.91 1.136 1.155 1.009 228.91 0.22 0.242 0.234243.19 1.114 1.181 1.031 240.61 0.252 0.250 0.240

KI-II KI-II

Tcalib (K) (13-H) (15-H) Tcalib (K) (13-H) (13′-H)

(CD3)2CO C7D8

178.77 0.184 0.247 180.17 0.252 a

191.27 0.232 0.255 192.65 0.319 a

203.57 0.241 0.278 203.57 0.503 0.47215.68 0.252 0.325 215.68 0.625 0.59227.6 0.265 0.350 227.6 0.727 0.657241.90 0.287 0.339 243.19 0.827 0.763

aThe signal is overlapped.

Table III. Equilibrium ConstantsKI-II for Lactone2: CD2Cl2, CD3OD, (CD3)2CO, and C7D8

KI-II KI-II

Tcalib (K) (13-H) (6-H) (7-H) Tcalib (K) (13-H) (6-H) (7-H)

CD2Cl2 CD3OD181.57 1.937 1.861 1.748 180.17 0.128 0.151 0.142192.65 2.190 2.128 1.954 192.65 0.149 0.131 0.123203.57 2.139 2.065 1.918 204.93 0.183 0.171 0.149214.39 2.220 2.122 1.959 217.02 0.195 0.204 0.167227.6 2.150 2.136 1.958 228.91 0.234 0.246 0.208244.47 2.397 2.171 2.00 241.9 0.259 0.257 0.227

KI-II KI-II

Tcalib (K) (13-H) (15-H) Tcalib (K) (13-H) (13′-H)

(CD3)2CO C7D8

180.17 0.208 0.237 180.17 0.616 a

191.27 0.240 0.256 192.65 0.803 a

203.57 0.285 0.308 204.93 1.002 1.027217.02 0.341 0.336 215.68 1.128 1.161228.91 0.375 0.352 228.91 1.34 1.374241.90 0.355 0.394 241.90 1.426 1.505

aThe signal is overlapped.

Table IV. Thermodynamic Parameters of I⇀↽II Process for Lactones1 and2 Obtained from Linear Plot of Eq. (1)

CD2Cl2 CD3OD (CD3)2CO C7D8

Compd 1H◦a 1S◦b 1H◦ 1S◦ 1H◦ 1S◦ 1H◦ 1S◦

1 0.52± 0.07 2.41± 0.35 0.9± 0.05 0.97± 0.26 0.53± 0.14 −0.07± 0.69 1.61± 0.13 6.33± 0.612 0.20± 0.06 2.40± 0.28 0.91± 0.10 0.93± 0.50 0.78± 0.07 1.37± 0.32 1.21± 0.07 5.85± 0.33

a1H◦ values are given in kcal-mol−1.b1S◦ values in cal kcal mol−1.

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Conformational Analysis of Guaianolide-Type Sesquiterpene Lactones 241

Table V. 1G◦ Values Calculated by Eq. (1) from Data Presented inTable IV

1G◦ (kcal-mol−1)b

Compound T (K)a CD2Cl2 CD3OD (CD3)2CO C7D8

183 0.08 0.72 0.54 0.451 243 −0.07 0.66 0.55 0.07

298 −0.2 0.61 0.55 −0.28183 −0.24 0.74 0.53 0.14

2 243 −0.38 0.68 0.45 −0.21298 −0.51 0.63 0.37 −0.53

aValues relate to the ends of the experimental temperature interval andto room temperature.

bCalculated deviations are less than 5%.

Kinetic Parameters of the ConformationalExchange I→ II

The conformer interconversion of compounds1 and2 was also investigated by the complete band-shape anal-ysis over a temperature interval of 233 to 313 K. The I→II system was described as one-spin two-sites nonmutualexchange and 6-H triplets were simulated. In performingthese simulations, we assumed that the NMR line widths(in the absence of exchange broadening) and the activa-tion energies have negligible temperature dependence. Atemperature-independent line width corresponding to aneffective relaxation timeT∗2 = 0.1 s was assumed for eachtype of proton. Our simulations did not account for dif-ferences in the relevant signal shapes between the con-formers due to several, weak spin-couplings. The chem-ical shifts and the line widths are extrapolated from theslow-exchange region to the region of faster exchange.While the relative populations of I and II conformers wereheld fixed at the calculated values, the interconversion rateconstants of the I→ II process (kI-II ) were allowed tovary, until qualitative agreement between simulated andexperimental line shape was achieved for each tempera-ture (Fig. 3).

The obtained rate constants clearly show that, withrising a temperature, factors favoring conformer II over Iare more pronounced for lactone2 (Table VI).

Table VI. Interconversion Rates of I⇀↽II Processes for Lactones1 and2 Gained by1H NMR Dynamic Simulation at All Experimental Temperatures

k (s−1)

Compound Reaction 233 K 243 K 253 K 263 K 268 K 273 K 278 K 283 K 288 K 293 K 303 K 313 K

1 I→II 0.01 4 15 47 49 73 120 200 270 500 900 1600II→Ia 0.01 3.4 11 34 35 53 87 145 195 362 652 1159

2 I→II 0.01 2 15 55 77 120 185 300 360 570 1400 4100II→Ia 0.01 0.8 6.44 24 33 51 79 129 154 244 600 1757

akII -I = kI-II [I]/[II]; [I] and [II] are the populations of conformers I and II, respectively, estimated on the integral values of 6-H NMR signals.

Fig. 3. Experimental (CD2Cl2, noisy line) and simulated (smooth line)6-H triplets of the conformers I and II, at selected temperatures: (a)compound1; (b) compound2.

The activation energiesEa were calculated from theArrhenius Eq. (2):

k = Ae−(Ea/RT) (2)

and its linear form (lnk = ln A− Ea/RT) allowed forthe linear simple least-squares analysis illustrated inFig. 4.

The obtainedEa values are 12.78± 0.35 and 15.11± 0.64 kcal-mol−1, for compounds1 and2, respectively.

Geometry Optimization

Resemblance between lactones1 and 2 was con-firmed by the results of their geometry optimization. For

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Fig. 4. Linear plot of the rate constants for I→II process versus temperature according to Eq. (2): (a) compound1, r = −0.997;(b) compound2, r = −0.992.

both,1 and2, semiempirical PM3 calculations predictedfour distinctive conformations that we assigned: I, Ia, II,and III (Fig. 5).

The essential difference between resultant geome-tries is conformation of the heptane ring. Conformers Iand Ia are so similar that NMR spectra would hardly dis-tinguish them: cycloheptane is a chair in I and slightly

Fig. 5. Low energy conformations of guaianolides1 (R1 = Ac, R2 = OAc) and2 (R1 = i -But, R2 = OAc)found by the PM3 semiempirical calculations.

distorted chair in Ia (in both, I and Ia, OR1 and OH arepseudoequatorial, while OR2 and 10-CH3 are pseudoax-ial). Heptane ring adopts twisted chair geometry in con-former II, i.e., bulky OR2 and 10-CH3 are pseudoequa-torial and OR1 is in a position where both orientations(eq./ax.) are isoenergetic (C-8 atom of lactones1–5 corre-sponds to C-1 atom of unsubstituted heptane ring) [5]. In

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Conformational Analysis of Guaianolide-Type Sesquiterpene Lactones 243

conformer III, cycloheptane has distorted boat geometry(Fig. 5).

Analyzing low-temperature 1D (1H and13C) NMRspectra and characteristic correlations in 2D NMR spectra(DQF COSY, PS NOESY, HSQC, HMBC) of compounds1 [2a] and 2 [2b], one could conclude that 10-CH3 ispseudoaxial in conformer I (δc = 21.7 ppm; nOe correla-tions: 14-H/9-H, 14-H/8-H, and 14-H/6-H for both,1 and2), and pseudoequatorial in conformer II (δc = 27.9 ppmfor 1, δc = 27.6 ppm for2; nOe correlations: 9-H/6-H, 9-H/2-Hβ, and 1-H/14-H for both,1 and2). Consequently,cycloheptane is in a chair conformation in conformer Iand in a twisted chair conformation in conformer II.Characteristic differences between conformers I and IIof lactones1 and 2 are vicinal coupling constantsJ7,8

and J8,9 in their 1H NMR spectra. In an attempt toform a relation between experimentally established andcalculated geometries, we applied three versions of theKarplus equation [6] to the corresponding dihedral angles.Equation (3) (i.e., the reparametrized standard Karplusequation [6b]) gave the best concordance with experi-mental results (Table VII;8 relates to H-C(7)-C(8)-Hand to H-C(8)-C(9)-H forJ7,8 andJ8,9, respectively).

J = Acos28+ B cos8+ C (A = 7.76,

B = −1.0,C = 1.40) (3)

Comparing the geometries (Table VII) it is obviousthat experimentally deduced conformation I well agreeswith theoretically calculated conformation I (or I+ Ia) andthat experimental conformation II suits calculated confor-mation II for both1 and2. Data from Table VII confirmedthat conformer III should be ignored.

The geometry optimization also revealed that, if10-OH hydroxyl proton and carbonyl O-atom of anOR-moiety (at C-8 or C-9) are suitably oriented, there arepossibilities for intramolecular hydrogen bonding only in

Table VII. Calculated and Experimental3JHH (in Hz) for Conformersof Compounds1 and2

Jcalc. Jaexp.

Compound Conformer J7,8 J8,9 J7,8 J8,9

I 9.96 2.21 Ia 9.19 2.74 11.1 2.6

II 9.19 3.83 9.1 5.1III 4.17 2.3I 9.98 2.15

2 Ia 9.17 2.61 10.9 2.5II 9.45 4.3 8.8 5.0III 5.63 2.71

aCD2Cl2, T < 220 K.

rotamers of conformation II (Fig. 6). Moreover, it shouldbe emphasized that 10-OH is pseudoequatorial in con-former I, and, thus, more available for intermolecular hy-drogen bonding with solvent molecules. Consequently,this interaction could be the essential one for, experimen-tally observed, domination of the conformer I in polarsolvents [CD3OD and (CD3)2CO; Table I]. At higher tem-peratures, this interaction weakens and the population ofenergetically less stable conformer II increases. A lackof intermolecular hydrogen bonds allows more expres-sive ring inversion in nonpolar solvents. Relatively large1S◦ values (Table IV), as well as the possibility of in-tramolecular hydrogen bonding (Fig. 6) are the factorsacting in the same direction—favoring conformer II innonpolar solvents (CD2C12 and C7D8) more than in polarones (particularly for2 in CD2C12), as the NMR experi-ments initially revealed (Table I).

Results of the Geometry Optimizationof 9α-Hydroxycumambrine A (3),9α-Acetoxy-cumambrine B (4), andCumambrine B (5) Regarding NMR Data

The inseparable, isomeric guaianolides3 and 4(Fig. 1) were isolated as a mixture with ratio3/4= 15/85during previous investigations [2c]. In the1H NMR spec-trum of the mixture (r.t., CDC13), compound3 appearswith sharp, well-resolved signals, while signals of lactone4 are broadened (similar, but less than for compounds1and 2). The 1H NMR data were assigned by means ofDQF COSY, PS NOESY, HSQC, and HMBC spectra ofthe mixture. The nOe correlations and the other NMR datasuggested that lactone3 adopts conformation I related tolactones1 and2 (vide supra). Slightly broadened NMRlines of lactone4, typical for a conformational exchangeof a medium rate (on NMR time scale), indicate that inconformational mixture dominates conformer II, alreadydefined for compounds1 and2 (vide supra).

Cumambrine B5 could be described as the leaststructural factor contained in compounds1–4 (Fig. 1). Ac-cording to the reported NMR data [7], lactone5 adopts arigid conformation with heptane ring in twisted chair ge-ometry.

Very similar to lactones1 and2, optimization of ge-ometries3–5 by PM3 semiempirical method resulted infour conformations, I, Ia, II, and III (Fig. 5) and again,regarding calculated and measured relevant coupling con-stants, conformer III could be disregarded (Tables VIIIand IX). Data from Table VIII confirm that lactone3, mostlikely, prefers conformation I (or I+ Ia), while conforma-tion of lactone4 closely relates to calculated geometry II

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Fig. 6. PM3 calculated rotamers of conformation II with intramolecular hydrogen bonds: (a) compound1;(b) compound2.

(Fig. 5, R1 = Ac, R2 = OH and R1 = H, R2 = OAc for 3and4, respectively).

In the case of cumambrine B5, comparison of calcu-lated geometries with experimental results is of little help,since experimental coupling constants pretty well agreewith those calculated for all three significant conformers(Table IX).

However, calculated geometry II (Fig. 5, R1 = R2 =H), with cycloheptane in twisted chair conformation, was

Table VIII. Relevant3JHH (in Hz) for Lactones3 and4

Jacalc. Jb

exp.

Compound Conformer J7,8 J8,9 J7,8 J8,9

I 9.91 1.95Ia 9.31 2.5

3 10.9 2.8II 9.3 4.4III 4.5 2.6I 10.1 2.4Ia 9.92 3.1

4 8.6 4.5II 9.29 4.23III 5.5 2.5

aBy Eq. (3).bCDCl3, r.t.

deduced also from the geminal couplings and paramag-netic shift of exomethylene protons [1f, 7a], so it shouldbe the most probable conformation of cumambrine B5.

CONCLUSIONS

The results of the conformational analysis obtainedby low-temperature NMR experiments, also supportedby PM3 semiempirical calculations, indicate that con-formational exchange of compounds1 and2 occurs be-tween two distinctive conformers, I and II, comprising

Table IX. Relevant3JHH (in Hz) for Lactone5

Jacalc.

Conformer J5,6 J6,7 J7,8

I 9.72 8.39 10.18Ia 9.91 7.7 9.73II 10.25 7.99 9.33III 10 8.52 5.7

Jbexp. 9.4 10 10

aBy Eq. (3).bRef. [7d].

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Conformational Analysis of Guaianolide-Type Sesquiterpene Lactones 245

“chair ⇀↽ twisted chair” interconversion of the heptanering. As1H NMR spectra show, the conformational equi-librium virtually depends on the solvent’s polarity and, toa lesser degree, on temperature. The thermodynamic pa-rameters of the exchange reaction I⇀↽ II and the resultsof the geometry optimization suggest that the position ofconformational equilibrium is attributable to two sets offactors: relative energies and intermolecular interactionsfavoring conformation I at low temperatures and entropyfactor and intramolecular interactions favoring conformerII at higher temperatures. The rate constants and Arrheniusactivation energies for the conversion I→ II of lactones1 and2 were deduced from dynamic NMR simulations.Relatively highEa values (∼13 and 15 kcal mol−1, for1 and2, respectively) if compared with interconversionbarriers of unsubstituted cycloheptane [5] are, probably,consequence of unfavorable van der Waals interactionsamong the substituents in the transition geometries on thepseudorotational coordinate of the I→ II reaction. TheEa is higher for lactone2, but not high enough to preventthe exchange process led by positive entropy difference.Slightly different conformational behavior of compounds1 and2 apparently originates from the single differencebetween them—bulkyi -But-group in lactone2, versusAc-group in1.

The calculated geometries of the guaianolides3–5are in a very good concordance with experimental (NMR)data, as well.

ACKNOWLEDGMENTS

The authors from Serbia and Montenegro are grate-ful to the serbian Ministry for Science, Technology andDevelopment for financial support. One of us (NT) thanks

the Mayo Foundation and DAAD for grants and Dr. ElineBasilio-Janke (FU Berlin) for technical assistance in someNMR experiments.

REFERENCES

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